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Description
The Dunderberg Shale and Halfpint members of the Nopah Formation (Upper Cambrian) are widespread lithologic units in the southern Great Basin and eastern Mojave Desert provinces of California and Nevada. Thirteen measured sections from this geographic area were described and the micro- and macro-fauna from seven of these sections were analyzed for age-dating and other paleontological data. From the lithostratigraphy, seven major lithofacies are distinguished: shale, stromatolitic biolithite, thrombolitic biolithite, oosparite, pelmatozoan bioclastic, chert nodule, and siltstone. From the paleontological data, two "time lines" were distinguished, the informal Chancelloria-Hintzespongia sponge concurrent range biozone (=lower Dunderbergia Trilobite Biozone, Upper Dresbachian Stage) and the Dunderbergia-Elvinia Trilobite Assemblage Biozone boundary (Dresbachian-Franconian Stage boundary). Also from the paleontological data, twenty-four trilobite, seven brachiopod, sixteen conodont, three sponge, one gastropod, and one ostracod species were identified. Interpretation of the sedimentary features and paleontology of the seven lithofacies has provided two paleogeographic reconstructions. In general, the study area lies in the transition zone between the outer detrital belt and the middle carbonate belt of Palmer. During the earliest Dunderbergia Biozone a major transgression and influx of terrigenous material occurred. The terrigenous material (shale lithofacies) was deposited under a variety of environments from shallow hypersaline conditions (southeast) to normal "subtidal" marine conditions (northwest). During the latest Dunderbergia Biozone and earliest Elvinia Biozone, regression occurred and algal barrier reefs, patch reefs, and lagoonal deposits formed. The algal banks (thrombolitic biolithite lithofacies) blocked terrigenous influx from offshore areas and produced a lagoon which supported a moderately rich, normal marine fauna that existed in a pelmatozoan bioclastic and chert nodule lithofacies. Seaward of the algal barrier reefs, the unfossiliferous siltstone lithofacies was deposited in shallow, moderately hypersaline conditions.
